1. Field of the Invention
The present invention relates to a liquid crystal display device provided with a direct backlight and more particularly to a liquid crystal display device able to narrow a picture frame and make the luminance of a liquid crystal panel uniform.
2. Description of the Related Art
Liquid crystal display devices are used to constitute a monitor in a thin TV receiver, a stand-alone type monitor, and a monitor in a notebook-size personal computer. A conventional liquid crystal display device is made up of a backlight, a liquid crystal panel which allows light projected from the backlight to pass therethrough selectively and form an image thereon, and a housing which houses therein the backlight and the liquid crystal panel. The types of backlights include an edge backlight wherein a light source such as a fluorescent lamp is disposed sideways of the housing and light is conducted to a liquid crystal panel by means of a light guide plate, and a direct backlight wherein a light source is disposed oppositely to a liquid crystal panel. The direct backlight, in comparison with the edge backlight, can improve the luminance of the liquid crystal panel and so is applied to a liquid crystal display device which constitutes, for example, a monitor in a high-definition type thin TV receiver.
The liquid crystal panel indicates a panel comprising a transparent substrate having switching elements such as TFTs (thin-film transistors) formed thereon, a transparent counter substrate opposed to the transparent substrate, and a liquid crystal sealed between the two substrates, with liquid crystal display device portions being disposed in a matrix form, and wherein drivers are connected respectively to a scanning line and a signal line both provided in each of the liquid crystal display device portions and a connecting substrate is connected to each of the drivers. The backlight comprises a light source such as a fluorescent lamp, a holder for holding the light source, and a diffuser for diffusing light emitted from the light source.
FIG. 1 is a partial sectional view showing the construction of a conventional liquid crystal display device. As shown in the same figure, the conventional liquid crystal display device is provided with a liquid crystal panel 22 for the display of image and a frame 21 of an L-shaped section, the frame 21 supporting the liquid crystal panel 22 while covering side portions of a surface of the liquid crystal panel 22 and constituting side portions of the whole of the liquid crystal display device. A frame-like cushion 38 is connected to the liquid crystal panel 22 and a chassis 23 is connected to the cushion 38. The cushion 38 is for preventing a glass substrate of the liquid crystal panel 22 from being damaged by contact thereof with the chassis 23. A side reflector 24 is connected to the chassis 23. The side reflector 24 supports a side portion of a diffuser 25 at an end portion thereof close to the liquid crystal panel 22. The diffuser 25 is a translucent plate of white color made of an acrylic resin for example and is disposed in parallel with the liquid crystal panel 22.
A single side reflector 24 is provided with plural lamp support bases 26 which are arranged in a row in a direction parallel to a surface of the liquid crystal panel 22. Each lamp support base 26 holds a low pressure-side end portion 27a of a lamp 27 which is a cold-cathode discharge tube. Plural lamps 27 are provided and are arranged in parallel. A return substrate 28 which extends in the arranged direction of the lamps 27 is attached to end portions of the lamp support bases 26 on the side opposite to the arranged side of the lamps 27. Lead wires 29 are drawn out axially of the lamps 27 from the low pressure-side end portions 27a of the lamps 27, then pass through the lamp support bases 26, reach the return substrate 28 and are connected to the return substrate 28 by solder 28b formed on a surface 28a of the return substrate 28. The surface 28a is perpendicular to a display surface of the liquid crystal panel 22. The return substrate 28 interconnects the lead wires 29 which are connected to the low pressure-side end portions 27a of the lamps 27.
A single cable (not shown) is connected at one end thereof to the return substrate 28 and is also connected at an opposite end thereof to an inverter substrate (not shown), which inverter substrate is connected to high pressure-side end portions (not shown) of the lamps 27. For preventing contact of the return substrate 28 with the frame 21, which would cause a short, there is formed a gap 31 between the return substrate 28 and the frame 21 and an insulating sheet 32 is attached to an inner surface of the frame 21. Further, a bottom reflector 30 is provided in an opposed relation to the diffuser 25 with the lamps 27 therebetween. The bottom reflector 30 is for reflecting light from the lamps 27 toward the liquid crystal panel 22.
Ineffective light emitting portions 27b are provided respectively at both end portions of each lamp 27 which is a cold-cathode discharge tube. The portion between the ineffective light emitting portions 27b at both ends is an effective light emitting portion 27c. The ineffective light emitting portions 27b of each lamp 27 are portions of a cold-cathode discharge tube within which are disposed electrodes (not shown). The ineffective light emitting portions 27b also emit light, but the emitted light is weak and not uniform. The length of each ineffective light emitting portion 27b is usually about 7 mm although it differs depending on lamp manufacturers. In this conventional liquid crystal display device, as shown in FIG. 1, the ineffective light emitting portions 27b are mostly covered with the side reflector 24 and the lamp support bases 26.
In the conventional liquid crystal display device, a picture frame area 34 which does not display images is formed around a display area 33 which displays images through the liquid crystal panel 22. The picture frame area 34 is defined by the ineffective light emitting portions 27b, lamp support bases 26, return substrate 28, gap 31, and frame 21 with an insulating sheet 32 affixed to the inner surface thereof.
Recently, for the liquid crystal display device there has been a request for a decrease of its size and an increase in size of the display area. To meet this request it is required to form the picture frame area as narrow as possible. In the conventional liquid crystal display device, however, as shown in FIG. 1, the ineffective light emitting portions 27b are present at both ends of each lamp 27, and between the lamps 27 and the frame 21 there are provided lamp support bases 26, return substrate 28 and gap 31, further, the insulating sheet 32 is affixed to the inner surface of the frame 21. Thus, a limit is encountered in narrowing the picture frame.
FIG. 2 is a partial sectional view showing the construction of another conventional liquid crystal display device. In the liquid crystal display device shown in FIG. 2, the length of each lamp 27 is set shorter than the liquid crystal panel 22 in order to make the picture frame area 34 as narrow as possible. Further, the chassis 23, side reflector 24 and lamp support bases 26 used in the liquid crystal display device of FIG. 1 are rendered integral and replaced by a chassis with reflector 36. The other construction of the conventional liquid crystal display device shown in FIG. 2 is the same as that of the liquid crystal display device shown in FIG. 1.
However, this conventional liquid crystal display device shown in FIG. 2 involves the following problems. In the liquid crystal display device, the length of each lamp 27 is set shorter than the liquid crystal panel 22 in order to make the picture frame area 34 as narrow as possible. As a result, although the picture frame area 34 of the liquid crystal display device shown in FIG. 2 is narrower than that of the liquid crystal display device shown in FIG. 1, the ineffective light emitting portions 27b of each lamp 27 are not wholly covered with the chassis with reflector 36 and protrude between the chassis with reflector 36. In FIG. 2, the length of each ineffective light emitting portion 27b thus protruding between the chassis with reflector 36 is assumed to be A. As a result, when displaying an image in the display area 33, the effective light emitting portion 27c of each lamp 27 becomes farther from the chassis with reflector 36 by the length A, thus giving rise to the problem that side portions of the display area 33 become dark.
Moreover, in the conventional liquid crystal display devices shown in FIGS. 1 and 2, the surface 28a of the return substrate 28 is perpendicular to the display surface of the liquid crystal panel 22, so when assembling the liquid crystal display device, the solder 28b is applied to the return substrate surface 28a while erecting the structure comprising the lamps 27, lamp support bases 26 and side reflector 24 so that the axes of the lamps 27 become vertical. Thus, not only the assembling work becomes difficult and the working efficiency lowers but also the assembling work becomes more dangerous.